1. Field of the Invention
The present invention relates to an optical unit or package, and more particularly to an optical unit or package which includes a substrate, and at least one optical element or part, such as electrooptical or Pockel's elements, magnetooptical or Faraday elements, and lenses, which is fixedly supported on the substrate.
2. Discussion of the Prior Art
Generally, optical elements or components such as electrooptical or magnetooptical elements and lenses are secured on a suitable support member or substrate by a suitable fixing means such as synthetic resin adhesives or screws, so that the optical elements may serve their function in the appropriate fields of application. Such a known method or structure for supporting the optical elements suffers from strain, deformation of the optical elements, or variation in their refractive index, due to stresses which may arise from a difference in the thermal expansion coefficient between the optical elements and the substrate, which are inevitably subject to varying ambient temperature. When the optical elements are electrooptical or Pockel's elements or magnetooptical or Faraday elements, the thermal stresses indicated above entail significant changes in the optical properties of the elements. When the optical elements are bonded to the substrate with an adhesive, the properties of the optical elements may be changed due to stresses caused by contraction of the adhesive during bonding of the elements to the substrate.
An example of an optical sensor using a conventional optical unit or package is shown in FIG. 1. The optical sensor, which includes the optical unit, further includes a light source 1 for producing a light beam incident upon the optical unit, and a light receiving element or photo-detector 7 adapted to receive an optical output of the optical unit in question. The optical unit includes an optical fiber 2, an electrooptical crystal 3, a polarizer 4a, an analyzer 4b, a quarter waveplate (.lambda./4 plate) 5 and rod lenses 6. Reference numeral 8 designates a pair of light-transparent electrodes disposed on both sides of the faces of the electrooptical crystal 3. The optical elements 3, 4a, 4b, 5 and 6 are bonded to one of opposite major surfaces of a substrate 10 by a suitable synthetic resin adhesive indicated at 9 in FIG. 1, whereby the optical elements are fixedly supported as a unit by the substrate 10.
As is well known in the art, an optical sensor is operated utilizing the electrooptical or Pockel's effect, or the magnetooptical or Faraday effect. The electrooptical or magnetooptical effect is defined as a phenomenon wherein optical properties of a transparent crystal are changed when the crystal is placed in an applied electric or magnetic field. In the optical sensor of FIG. 1, wherein a single crystal of lithium niobate (LiNbO.sub.3) or Bi.sub.12 SiO.sub.20 is used as the electrooptical crystal (Pockel's element) 3, the light beam produced by the light source 1 is linearly polarized by the polarizer 4a, and the linearly polarized light is elliptically polarized by the electrooptical crystal 3. The elliptically polarized light is passed through the quarter waveplate 5 and the analyzer 4b, and is received by the light detector 7 via the rod lens 6. The quantity of the light beam or optical power emitted from the optical unit is determined by the ellipticity of the elliptically polarized light emitted from the electrooptical crystal 3. Namely, the quantity of the elliptically polarized light received by the photo-detector 7 is changed as a function of a voltage applied across the electrooptical crystal 3 via the electrodes 8. Therefore, the voltage applied to the crystal 3 can be determined by measuring the quantity of the elliptically polarized light, i.e., the output of the optical unit of the optical sensor. In the thus constructed optical sensor, however, the output of the optical unit tends to be changed due to a variation in the ambient temperature and a consequent variation in the optical properties of the electrooptical crystal 3 bonded to the substrate 10.
In other types of optical units or packages using a magnetooptical or Faraday crystal, prism lens, reflector or any optical element other than an electrooptical element as discussed above, the optical element suffers from strain or deformation due to stresses arising from a difference in the coefficient of thermal expansion between the optical element, and a substrate made of a ceramic or metal material, for example, on which the optical element is secured or bonded. That is, a change in the ambient temperature of the optical sensor may cause a variation in the optical properties of the optical element, and a change in the direction of propagation of a light beam through the optical element. Further, repeated changes in the ambient temperature may even cause a fatigue of the bond between the optical element and the substrate, eventually resulting in a failure or fracture of the bond.